Closed circuit breathing apparatus which is adapted to be worn on the back (or front) of the user is well known in the prior art and typical examples are U.S. Pat. Nos. 3,863,629; 4,567,889 and U.K. Pat. No. 992,428. This apparatus may typically be worn by personnel fighting fires, although it has many other applications. In a closed circuit apparatus the user recycles his exhalation gas after the carbon dioxide has been removed and the oxygen consumed by the user has been made up. Thus, in the prior art devices referred to, there is a closed gas circulating circuit including a carbon dioxide absorber or scrubber, and a face mask which is interconnected with the CO2 scrubber by inhalation and exhalation tubes or passageways, these tubes being provided with suitable check valves. It is also a feature of the above prior art to provide a make-up source of breathing gases, typically pure oxygen, as well as a counterlung or the equivalent in the form of a breathing bag in which purified gases are stored prior to inhalation so that the wearer of the apparatus will typically have sufficient purified gases for all but the largest inhalations.
As face mask seals sometimes leak it is frequently desirable to provide a positive pressure system so that if there is leakage about the face mask seal, irrespirable gases will not be inhaled. This is typically done by providing a positive pressure to the counterlung as taught in U.S. Pat. No. 4,567,889.
While the above devices apparently perform in a satisfactory manner for their intended purposes, such devices require the wearer to exert greater inhalation and exhalation efforts than he would normally be required in an open atmosphere. While such increased breathing effort cannot be totally eliminated, it is desirable to minimize the increased effort as much as possible. Thus, by providing a counterlung on the downstream side of the carbon dioxide scrubber, inhalation effort is typically reduced to an acceptable level to the extent that a sufficient volume of breathable gases are contained in the counterlung. However, if this is not the case, and if it is then necessary to draw upon the breathable gases being processed through the carbon dioxide scrubber, the inhalation effort will substantially increase. In order to reduce exhalation breathing resistance it has been proposed to utilize an ejector pump to assist the user's respiration by pumping exhaled gas through the scrubber at a relatively constant volumetric flow rate, and this design feature is also shown in U.S. Pat. No. 4,567,889. However, at exhalation rates greater than the flow rate of the pump, increased breathing resistance is still encountered up to the limit established by an overpressure valve.
U.S. Pat. No. 3,815,592 shows breathing bags to either side of a carbon dioxide absorber and source of oxygen. In this device the pressure in the exhalation side of the circuit is greater than the pressure in the inhalation side, leading to greater exhalation effort at all times. In addition, if the inhalation accumulator cannot supply all the needed gas during inhalation, substantially increased inhalation effort will be encountered.
U.S. Pat. No. 2,106,393 discloses an emergency breathing apparatus having a common inhalation-exhalation breathing bag. While this form of device will have a lower breathing resistance, it will accumulate excessive carbon dioxide.